Cellular telephone system with free space millimeter wave trunk line

ABSTRACT

A wireless cellular communication system in which groups of cellular base stations communicate with a central office via a narrow-band millimeter wave trunk line. The transceivers are equipped with antennas providing beam divergence small enough to ensure efficient spatial and directional partitioning of the data channels so that an almost unlimited number of transceivers will be able to simultaneously use the same millimeter wave spectrum. In a preferred embodiment the trunk line communication link operates within the 92 to 95 GHz portion of the millimeter spectrum. A large number of base stations are each allocated a few MHz portion of a 900 MHz bandwidth of the millimeter wave trunk line. A first transceiver transmits at a first bandwidth and receives at a second bandwidth both within the above spectral range. A second transceiver transmits at the second bandwidth and receives at the first bandwidth. Antennas are described to maintain beam directional stability to less than one-half the half-power beam width. In a preferred embodiment the first and second spectral ranges are 92.3-93.2 GHz and 94.1-95.0 GHz and the half power beam width is about 0.36 degrees or less. Thus, in this system the low frequency band width is efficiently utilized over and over again by dividing a territory into small cells and using low power antenna. And a higher frequency bandwidth is efficiently utilized over and over again by using transmitting antennae that are designed to produce very narrow beams directed at receiving antennae. In a preferred embodiment cellular base stations are prepackaged for easy quick installation at convenient locations such as the tops of commercial buildings.

[0001] The present invention relates to wireless communications linksand specifically to high data rate point-to-point links. Thisapplication is a continuation-in-part application of Ser. No. 09/847,629filed May 2, 2001, Ser. No. 09/872,542 filed Jun. 2, 2001, Ser. No.09/872,621 filed Jun. 2, 2001, and Ser. No. 09/882,482 filed Jun. 14,2001.

BACKGROUND OF THE INVENTION Local Wireless Radio Communication

[0002] Local wireless communication services represent a very rapidlygrowing industry. These services include paging and cellular telephoneservices. The cellular telephone industry currently is in its secondgeneration with several types of cellular telephone systems beingpromoted. The cellular market in the United States grew from about 2million subscribers and $2 million in revenue in 1988 to more than 60million subscribers about $30 billion in revenue in 1998 and the growthis continuing in the United States and also around the world as theservices become more available and prices decrease.

[0003]FIG. 1 describes a typical cellular telephone system. A cellularservice provider divides its territory up into hexagonal cells as shownin FIG. 1. These cells may be about 5 miles across, although in denselypopulated regions with many users these cells may be broken up into muchsmaller cells called micro cells. This is done because cellularproviders are allocated only a limited portion of the radio spectrum.For example, one spectral range allocated for cellular communication isthe spectral range: 824 MHz to 901 MHz. (Another spectral rangeallocated to cellular service is 1.8 GHz to 1.9 GHz) A provideroperating in the 824-901 MHz range may set up its system for thecellular stations to transmit in the 824 MHz to 851 MHz range and toreceive in the 869 MHz to 901 MHz range. The transmitters both at thecellular stations and in devices used by subscribers operate at very lowpower (just a few Watts) so signals generated in a cell do not provideinterference in any other cells beyond immediate adjacent cells. Bybreaking its allocated transmitting spectrum and receive spectrum inseven parts (A-G) with the hexagonal cell pattern, a service providercan set up its system so that there is a two-cell separation between thesame frequencies for transmit or receive, as shown in FIG. 1. A one-cellseparation can be provided by breaking the spectrum into three parts.Therefore, these three or seven spectral ranges can be used over andover again throughout the territory of the cellular service provider. Ina typical cellular system each cell (with a transmit bandwidth and areceive bandwidth each at about 12 MHz wide) can handle as many as about1200 two-way telephone communications within the cell simultaneously.With lower quality communication, up to about 9000 calls can be handledin the 12 MHz bandwidth. Several different techniques are widely used inthe industry to divide up the spectrum within a given cell. Thesetechniques include analog and digital transmission and severaltechniques for multiplexing the digital signals. These techniques arediscussed at pages 313 to 316 in The Essential Guide toTelecommunications, Second Edition, published by Prentice Hall and manyother sources. Third generation cellular communication systems promisesubstantial improvements with more efficient use of the communicationspectra.

Other Prior Art Wireless Communication Techniques Point-to-Point andPoint-to-Multi-Point

[0004] Most wireless communication, at least in terms of datatransmitted is one way, point to multi-point, which includes commercialradio and television. However, there are many examples of point-to-pointwireless communication. Cellular telephone systems, discussed above, areexamples of low-data-rate, point-to-point communication. Microwavetransmitters on telephone system trunk lines are another example ofprior art, point-to-point wireless communication at much higher datarates. The prior art includes a few examples of point-to-point lasercommunication at infrared and visible wavelengths.

Information Transmission

[0005] Analog techniques for transmission of information are stillwidely used; however, there has recently been extensive conversion todigital, and in the foreseeable future transmission of information willbe mostly digital with volume measured in bits per second. To transmit atypical telephone conversation digitally utilizes about 5,000 bits persecond (5 Kbits per second). Typical personal computer modems connectedto the Internet operate at, for example, 56 Kbits per second. Music canbe transmitted point to point in real time with good quality using MP3technology at digital data rates of 64 Kbits per second. Video can betransmitted in real time at data rates of about 5 million bits persecond (5 Mbits per second). Broadcast quality video is typically at 45or 90 Mbps. Companies (such as line telephone, cellular telephone andcable companies) providing point-to-point communication services buildtrunk lines to serve as parts of communication links for theirpoint-to-point customers. These trunk lines typically carry hundreds orthousands of messages simultaneously using multiplexing techniques.Thus, high volume trunk lines must be able to transmit in the gigabit(billion bits, Gbits, per second) range. Most modem trunk lines utilizefiber optic lines. A typical fiber optic line can carry about 2 to 10Gbits per second and many separate fibers can be included in a trunkline so that fiber optic trunk lines can be designed and constructed tocarry any volume of information desired virtually without limit.However, the construction of fiber optic trunk lines is expensive(sometimes very expensive) and the design and the construction of theselines can often take many months especially if the route is over privateproperty or produces environmental controversy. Often the expectedrevenue from the potential users of a particular trunk line underconsideration does not justify the cost of the fiber optic trunk line.Digital microwave communication has been available since the mid-1970's.Service in the 18-23 GHz radio spectrum is called “short-haul microwave”providing point-to-point service operating between 2 and 7 miles andsupporting between four to eight T1 links (each at 1.544 Mbps).Recently, microwave systems operating in the 11 to 38 Ghz band have beendesigned to transmit at rates up to 155 Mbps (which is a standardtransmit frequency known as “OC-3 Standard”) using high order modulationschemes.

Data Rate and Frequency

[0006] Bandwidth-efficient modulation schemes allow, as a general rule,transmission of data at rates of about 1 to 8 bits per second per Hz ofavailable bandwidth in spectral ranges including radio wave lengths tomicrowave wavelengths. Data transmission requirements of 1 to tens ofGbps thus would require hundreds of MHz of available bandwidth fortransmission. Equitable sharing of the frequency spectrum between radio,television, telephone, emergency services, military and other servicestypically limits specific frequency band allocations to about 10%fractional bandwidth (i.e., range of frequencies equal to about 10% ofcenter frequency). AM radio, at almost 100% fractional bandwidth (550 to1650 GHz) is an anomaly; FM radio, at 20% fractional bandwidth, is alsoatypical compared to more recent frequency allocations, which rarelyexceed 10% fractional bandwidth.

Reliability Requirements

[0007] Reliability typically required for wireless data transmission isvery high, consistent with that required for hard-wired links includingfiber optics. Typical specifications for error rates are less than onebit in ten billion (10⁻¹⁰ bit-error rates), and link availability of99.999% (5 minutes of down time per year). This necessitates all-weatherlink operability, in fog and snow, and at rain rates up to 100 mm/hourin many areas. On the other hand cellular telephone systems do notrequire such high reliability. As a matter of fact cellular users(especially mobile users) are accustom to poor service in many regions.

Weather Conditions

[0008] In conjunction with the above availability requirements,weather-related attenuation limits the useful range of wireless datatransmission at all wavelengths shorter than the very long radio waves.Typical ranges in a heavy rainstorm for optical links (i.e., lasercommunication links) are 100 meters, and for microwave links, 10,000meters.

[0009] Atmospheric attenuation of electromagnetic radiation increasesgenerally with frequency in the microwave and millimeter-wave bands.However, excitation of rotational modes in oxygen and water vapormolecules absorbs radiation preferentially in bands near 60 and 118 GHz(oxygen) and near 23 and 183 GHz (water vapor). Rain, which attenuatesthrough large-angle scattering, increases monotonically with frequencyfrom 3 to nearly 200 GHz. At the higher, millimeter-wave frequencies,(i.e., 30 GHz to 300 GHz corresponding to wavelengths of 1.0 centimeterto 1.0 millimeter) where available bandwidth is highest, rainattenuation in very bad weather limits reliable wireless linkperformance to distances of 1 mile or less. At microwave frequenciesnear and below 10 GHz, link distances to 10 miles can be achieved evenin heavy rain with high reliability, but the available bandwidth is muchlower.

Setting Up Additional Cells in a Telephone System is Expensive

[0010] The cost associated with setting up an additional cell in a newlocation or creating a micro cell within an existing cell with prior arttechniques is in the range of about $650,000 to $800,000. (See page 895Voice and Data Communication Handbook, Fourth Edition, published byMcGraw Hill.) These costs must be recovered from users of the cellularsystem. People in the past have avoided use of their cellular equipmentbecause the cost was higher that their line telephones. Recently, costshave become comparable.

The Need

[0011] Therefore, a great need exists for techniques for adding, at lowcost, additional cells in cellular communication systems.

SUMMARY OF THE INVENTION

[0012] The present invention provides a wireless cellular communicationsystem in which groups of cellular base stations communicate with acentral office via a narrow-beam millimeter wave trunk line. Thetransceivers are equipped with antennas providing beam divergence smallenough to ensure efficient spatial and directional partitioning of thedata channels so that an almost unlimited number of point-to-pointtransceivers will be able to simultaneously use the same millimeter wavespectrum. In a preferred embodiment the trunk line communication linkoperates within the 92 to 95 GHz portion of the millimeter spectrum. Alarge number of base stations are each allocated a few MHz portion of a900 MHz bandwidth of the millimeter wave trunk line. A first transceivertransmits at a first bandwidth and receives at a second bandwidth, bothwithin the above spectral range. A second transceiver transmits at thesecond bandwidth and receives at the first bandwidth. Antennas aredescribed to maintain beam directional stability to less than one-halfthe half-power beam width. In a preferred embodiment the first andsecond spectral ranges are 92.3-93.2 GHz and 94.1-95.0 GHz and the halfpower beam width is about 0.36 degrees or less. Thus, in this system thelow frequency bandwidth is efficiently utilized over and over again bydividing a territory into small cells and using low power antenna. Thehigher frequency bandwidth is efficiently utilized over and over againby using transmitting antennae that are designed to produce very narrowbeams directed at receiving antennae. In a preferred embodiment cellularbase stations are prepackaged for easy quick installation at convenientlocations such as the tops of commercial buildings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a sketch showing a prior art cellular network.

[0014]FIG. 2 is a sketch showing features of a single prior art cell.

[0015]FIG. 3 is a sketch of a preferred embodiment of the presentinvention.

[0016]FIG. 4 demonstrates up conversion from cell phone frequencies totrunk line frequencies.

[0017]FIG. 5 demonstrates down conversion from trunk line frequencies tocell phone frequencies.

[0018]FIG. 6 is a block diagram showing the principal components of aprepackaged cellular base station designed for roof-top installation.

[0019]FIG. 7 is a schematic diagram of a millimeter-wave transmitter ofa prototype transceiver system built and tested by Applicants.

[0020]FIG. 8 is a schematic diagram of a millimeter-wave receiver of aprototype transceiver system built and tested by Applicants.

[0021]FIG. 9 is measured receiver output voltage from the prototypetransceiver at a transmitted bit rate of 200 Mbps.

[0022]FIG. 10 is the same waveform as FIG. 9, with the bit rateincreased to 1.25 Gbps.

[0023]FIGS. 11A and 11B are schematic diagrams of a millimeter-wavetransmitter and receiver in one transceiver of a preferred embodiment ofthe present invention.

[0024]FIG. 12A and 12B are schematic diagrams of a millimeter-wavetransmitter and receiver in a complementary transceiver of a preferredembodiment of the present invention.

[0025]FIGS. 13A and 13B show the spectral diagrams for a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0026] A first preferred embodiment of the present invention comprises asystem of linked millimeter-wave radios which take the place of wire orfiber optic links between the cells of a cellular network. The use ofthe millimeter-wave links can eliminates the need to lay cable or fiber,can be installed relatively quickly, and can provide high bandwidthnormally at a lower cost than standard telecom-provided wires or cable.Since the millimeter-wave links simply up and down convert the signalfor point-to-point transmission, the data and protocols used by theoriginal signals are preserved, making the link ‘transparent’ to theuser. This embodiment supports a conventional system operating atstandard cellular telephone frequencies, but it is equally applicable toother, newer technologies such as 1.8 GHz to 1.9 GHz PCS systems.

[0027] A typical prior art cell phone base station transmits in the824-851 MHz band and receives in the 869-901 MHz band and is connectedmobile telephone switching office by wire connections which is in turnconnected to a central office via a high speed wired connection. Thecentral office performs call switching and routing. It is possible toreplace both wired links with a millimeter-wave link, capable ofcarrying the signals from several cellular base stations to the centraloffice for switching and routing, and then back out again to thecellular base stations for transmission to the users' cellular phonesand other communication devices. A millimeter-wave link with 1 GHz ofbandwidth will be capable of handling approximately 30 to 90 cellularbase stations, depending on the bandwidth of the base stations. Sincethe cellular base stations are typically within a few miles (or less formicro cells) of each other, the millimeter-wave link would form a chainfrom base station to base station, then back to the central office. FIG.3 illustrates the basic concept.

[0028] Most wireless computer networking equipment on the market todayis designed according to IEEE standards 802.11a and 802.11b thatdescribe a format and technique for packet data interchange betweencomputers. In this equipment the 802.11b formatted data is transmittedand received on one of eleven channels in the 2.4-2.5 GHz band and usesthe same frequencies for transmit and receive. Therefore, in thispreferred embodiment the cellular stations all operate on a slice of the2.4 to 2.5 GHz band using equipment built in accordance with the aboveIEEE standards. An up/down converter is provided up and down convert theinformation for transmittal on the millimeter wave p/down converter isdescribed below. Typically, base stations are organized in generallyhexagonal cells in groups of 7 cells as shown in FIG. 1. In order toavoid interference, each of the 7 cells operate at a different slice ofthe available bandwidth in each frequency slice is separated by twocells. If 3 different frequencies are group of 7 cells, there is aone-cell separation of frequencies.

Cellular Base Station Transmission Back to Central Office

[0029] Cell phone calls are received in the 824-851 MHz band at eachgroup of base stations, and up-converted to a 27 MHz slot of frequenciesin the 91-93 GHz band for transmission over the link back to the centraloffice. Each group of base stations is allocated a 27 MHz slice ofspectrum in the 91-93 GHz band as follows: Base Station Group NumberBase Station Frequency Trunk Line Frequency  1 824-851 MHz 91.000-91.027GHz  2 824-851 MHz 91.027-91.054 GHz  3 824-851 MHz 91.054-91.081 GHz .. . . . . . . . 30 824-851 MHz 91.783-91.810 GHz 31 824-851 MHz91.810-91.837 GHz 32 824-851 MHz 91.837-91.864 GHz

[0030]FIG. 4 shows a block diagram of a system that converts thecellular base station frequencies up to the millimeter-wave band fortransmission back to the central office. Each base station receives boththe cell phone frequencies within its cell, and the millimeter-wavefrequencies from the earlier base station in the chain. The cell-phonefrequencies are up-converted to a slot (of spectrum) in the 91-93 GHzband and added to the 91-93 GHz signals from the earlier base station upthe chain. The combined signals are then retransmitted to the next basestation in the chain. Each base station has a local oscillator set to aslightly different frequency, which determines the up-convertedfrequency slot for that base station. The local oscillator may bemultiplied by a known pseudo-random bit stream to spread its spectrumand to provide additional security to the millimeter-wave link.

[0031] At the telephone company central switching office, each 27 MHzslot of frequencies in the 91-93 GHz band is downconverted to thecellular telephone band. If a spread-spectrum local oscillator was usedon the millimeter-wave link, the appropriate pseudo random code must beused again in the downconverter's local oscillator to recover theoriginal information. Once the millimeter-wave signals are downconvertedto the cell phone band, standard cellular equipment is used to detect,switch, and route the calls.

Central Office Transmission to Cellular Base Stations

[0032] Cell phone calls leave the central office on a millimeter-wavelink and each group of cellular base stations downconverts a 32 MHzslice of the spectrum to the cell phone band for transmission to theindividual phones. The cellular base stations transmit (to the phones)in the 869-901 MHz band so each group of base stations requires a 32 MHzslice of the spectrum in the 91-93 GHz range on the millimeter wavelink. The 1.024 GHz will support 32 base stations. Each group of basestations is allocated a 32 MHz slice of spectrum in the 91-93 GHz bandas follows: Base station # Trunk Line Frequencies (link RX) converts toBase Station (cell TX) Base Station Group Number Trunk Line FrequencyBase Station Frequency  1 92.000-92.032 GHz 869-901 MHz  2 92.032-92.064GHz 869-901 MHz  3 92.064-92.096 GHz 869-901 MHz . . . . . . . . . 3092.928-92.960 GHz 869-901 MHz 31 92.960-92.992 GHz 869-901 MHz 3292.992-92.024 GHz 869-901 MHz

[0033] FIGS.5 shows a block diagram of a system that receivesmillimeter-wave signals from the central office and converts them to thecellular band for transmission by a cell base station. Each base stationreceives picks off the signals in its 32 MHz slice of the 91-93 GHzspectrum, down-converts this band to the cell phone band, and broadcastsit. The 91-93 GHz band is also retransmitted to the next base station inthe chain. Each base station a local oscillator set to a slightlydifferent frequency, which determines the 32 MHz wide slot (in the 91-93GHz band) that is assigned to that base station. If a spread-spectrumlocal oscillator was used on the up-conversion at the central office,then the appropriate pseudo random code must be used again in thedown-converter's local oscillator (at each base station) to recover theoriginal information.

[0034] At the telephone company central switching office calls aredetected, switched, and routed between the various cellular basestations and the landline network. Each group of cellular base stationsat the central office is represented by a 32 MHz wide slot of spectrum,which is up-converted to the 91-93 GHz band and sent out over apoint-to-point link to the chain of several base stations. The localoscillator used to up-convert the signals may be spread-spectrum toprovide additional security to the millimeter-wave link.

Prototype Demonstration of MM Wave T/R

[0035] A prototype demonstration of the millimeter-wave transmitter andreceiver useful for the present invention is described by reference toFIGS. 1 to 4. With this embodiment the Applicants have demonstrateddigital data transmission in the 93 to 97 GHz range at 1.25 Gbps with abit error rate below 10⁻¹².

[0036] The circuit diagram for the millimeter-wave transmitter is shownin FIG. 7. Voltage-controlled microwave oscillator 1, Westec ModelVTS133/V4, is tuned to transmit at 10 GHz, attenuated by 16 dB withcoaxial attenuators 2 and 3, and divided into two channels in two-waypower divider 4. A digital modulation signal is pre-amplified inamplifier 7, and mixed with the microwave source power intriple-balanced mixer 5, Pacific Microwave Model M3001HA. The modulatedsource power is combined with the un-modulated source power through atwo-way power combiner 6. A line stretcher 12 in the path of theun-modulated source power controls the depth of modulation of thecombined output by adjusting for constructive or destructive phasesummation. The amplitude-modulated 10 GHz signal is mixed with a signalfrom an 85-GHz source oscillator 8 in mixer 9 and high-pass filtered inwaveguide filter 13 to reject the 75 GHz image band. The resultant,amplitude-modulated 95 GHz signal contains spectral components between93 and 97 GHz, assuming unfiltered 1.25 Gbps modulation. A rectangularWR-10 wave guide output of the high pass filter is converted to acircular wave guide 14 and fed to a circular horn 15 of 4 inchesdiameter, where it is transmitted into free space. The horn projects ahalf-power beam width of 2.2 degrees.

[0037] The circuit diagram for the receiver is shown in FIG. 8. Theantenna is a circular horn 1 of 6 inches in diameter, fed from awaveguide unit 14R consisting of a circular W-band wave-guide and acircular-to-rectangular wave-guide converter which translates theantenna feed to WR-10 wave-guide which in turn feeds heterodyne receivermodule 2R. This module consists of a monolithic millimeter-waveintegrated circuit (MMIC) low-noise amplifier spanning 89-99 GHz, amixer with a two-times frequency multiplier at the LO port, and an IFamplifier covering 5-15 GHz. These receivers are available fromsuppliers such as Lockheed Martin. The local oscillator 8R is acavity-tuned Gunn oscillator operating at 42.0 GHz (Spacek ModelGQ410K), feeding the mixer in module R2 through a 6 dB attenuator 7. Abias tee 6 at the local oscillator input supplies DC power to receivermodule 2R. A voltage regulator circuit using a National SemiconductorLM317 integrated circuit regulator supplies +3.3V through bias tee 6. AnIF output of the heterodyne receiver module 2R is filtered at 6-12 GHzusing bandpass filter 3 from K&L Microwave. Receiver 4R which is an HPHerotek Model DTM 180AA diode detector, measures total received power.The voltage output from the diode detector is amplified in two-cascadedmicrowave amplifiers 5R from MiniCircuits, Model 2FL2000. The basebandoutput is carried on coax cable to a media converter for conversion tooptical fiber, or to a Bit Error-Rate Tester (BERT) 10R.

[0038] In the laboratory, this embodiment has demonstrated a bit-errorrate of less than 10⁻¹² for digital data transmission at 1.25 Gbps. TheBERT measurement unit was a Microwave Logic, Model gigaBERT. Theoscilloscope signal for digital data received at 200 Mbps is shown inFIG. 9. At 1.25 Gbps, oscilloscope bandwidth limitations lead to therounded bit edges seen in FIG. 10. Digital levels sustained for morethan one bit period comprise lower fundamental frequency components(less than 312 MHz) than those which toggle each period (622 MHz), sothe modulation transfer function of the oscilloscope, which falls offabove 500 MHz, attenuates them less. These measurement artifacts are notreflected in the bit error-rate measurements, which yield <10⁻¹² biterror rate at 1.25 Gbps.

Transceiver System

[0039] A preferred embodiment of the present invention is described byreference to FIGS. 11A to 13B. The link hardware consists of amillimeter-wave transceiver pair including a pair of millimeter-waveantennas and a microwave transceiver pair including a pair of microwaveantennas. The millimeter wave transmitter signal is amplitude modulatedand single-sideband filtered, and includes a reduced-level carrier. Thereceiver includes a heterodyne mixer, phase-locked intermediatefrequency (IF) tuner, and IF power detector.

[0040] Millimeter-wave transceiver A (FIGS. 11A and 11B) transmits at92.3-93.2 GHz as shown at 60 in FIG. 13A and receives at 94.1-95.0 GHzas shown at 62, while millimeter-wave transmitter B (FIGS. 12A and 12B)transmits at 94.1-95.0 GHz as shown at 64 in FIG. 13B and receives at92.3-93.2 GHz as shown at 66.

Millimeter Wave Transceiver A

[0041] As shown in FIG. 11A in millimeter-wave transceiver A, transmitpower is generated with a cavity-tuned Gunn diode 21 resonating at 93.15GHz. This power is amplitude modulated using two balanced mixers in animage reject configuration 22, selecting the lower sideband only. Thesource 21 is modulated at 1.25 Gbps in conjunction with Gigabit-Ethernetstandards. The modulating signal is brought in on optical fiber,converted to an electrical signal in media converter 19 (which in thiscase is an Agilent model HFCT-5912E) and amplified in preamplifier 20.The amplitude-modulated source is filtered in a 900 MHz-wide passbandbetween 92.3 and 93.2 GHz, using a bandpass filter 23 on microstrip. Aportion of the source oscillator signal is picked off with coupler 38and combined with the lower sideband in power combiner 39, resulting inthe transmitted spectrum shown at 60 in FIG. 13A. The combined signalpropagates with horizontal polarization through a waveguide 24 to oneport of an orthomode transducer 25, and on to a two-foot diameterCassegrain dish antenna 26, where it is transmitted into free space withhorizontal polarization.

[0042] The receiver unit at Station A as shown on FIGS. 11B1 and 11B2 isfed from the same Cassegrain antenna 26 as is used by the transmitter,at vertical polarization (orthogonal to that of the transmitter),through the other port of the orthomode transducer 25. The receivedsignal is pre-filtered with bandpass filter 28A in a passband from 94.1to 95.0 GHz, to reject back scattered return from the local transmitter.The filtered signal is then amplified with a monolithic MMWintegrated-circuit amplifier 29 on indium phosphide, and filtered againin the same passband with bandpass filter 28B. This twice filteredsignal is mixed with the transmitter source oscillator 21 using aheterodyne mixer-downconverter 30, to an IF frequency of 1.00-1.85 GHz,giving the spectrum shown at 39A in FIG. 13A. A portion of the IFsignal, picked off with coupler 40, is detected with integrating powerdetector 35 and fed to an automatic gain control circuit 36. Thefixed-level IF output is passed to the next stage as shown in FIG. 11B2.Here a quadrature-based (I/Q) phase-locked synchronous detector circuit31 is incorporated, locking on the carrier frequency of the remotesource oscillator. The loop is controlled with a microprocessor 32 tominimize power in the “Q” channel while verifying power above a setthreshold in the “I” channel. Both “I” and “Q” channels arelowpass-filtered at 200 MHz using lowpass filters 33A and 33B, and poweris measured in both the “I” and Q channels using square-law diodedetectors 34. The baseband mixer 38 output is pre-amplified and fedthrough a media converter 37, which modulates a laser diode source intoa fiber-optic coupler for transition to optical fiber transmissionmedia.

Transceiver B

[0043] As shown in FIG. 12A in millimeter-wave transceiver B, transmitpower is generated with a cavity-tuned Gunn diode 41 resonating at 94.15GHz. This power is amplitude modulated using two balanced mixers in animage reject configuration 42, selecting the upper sideband only. Thesource 41 is modulated at 1.25 Gbps in conjunction with Gigabit-Ethernetstandards. The modulating signal is brought in on optical fiber as shownat 80, converted to an electrical signal in media converter 60, andamplified in preamplifier 61. The amplitude-modulated source is filteredin a 900 MHz-wide passband between 94.1 and 95.0 GHz, using a bandpassfilter 43 on microstrip. A portion of the source oscillator signal ispicked off with coupler 48 and combined with the higher sideband inpower combiner 49, resulting in the transmitted spectrum shown at 64 inFIG. 13B. The combined signal propagates with vertical polarizationthrough a waveguide 44 to one port of an orthomode transducer 45, and onto a Cassegrain dish antenna 46, where it is transmitted into free spacewith vertical polarization.

[0044] The receiver is fed from the same Cassegrain antenna 46 as thetransmitter, at horizontal polarization (orthogonal to that of thetransmitter), through the other port of the orthomode transducer 45. Thereceived signal is filtered with bandpass filter 47A in a passband from92.3 to 93.2 GHz, to reject backscattered return from the localtransmitter. The filtered signal is then amplified with a monolithic MMWintegrated-circuit amplifier on indium phosphide 48, and filtered againin the same passband with bandpass filter 47B. This twice filteredsignal is mixed with the transmitter source oscillator 41 using aheterodyne mixer-downconverter 50, to an IF frequency of 1.00-1.85 GHz,giving the spectrum shown at 39B in FIG. 13B. A portion of the IFsignal, picked off with coupler 62, is detected with integrating powerdetector 55 and fed to an automatic gain control circuit 56. Thefixed-level IF output is passed to the next stage as shown on FIG. 12B2.Here a quadrature-based (I/Q) phase-locked synchronous detector circuit51 is incorporated, locking on the carrier frequency of the remotesource oscillator. The loop is controlled with a microprocessor 52 tominimize power in the “Q” channel while verifying power above a setthreshold in the “I” channel. Both “I” and “Q” channels arelowpass-filtered at 200 MHz using a bandpass filters 53A and 53B, andpower is measured in each channel using a square-law diode detector 54.The baseband mixer 58 output is pre-amplified and fed through a mediaconverter 57, which modulates a laser diode source into a fiber-opticcoupler for transition to optical fiber transmission media.

Very Narrow Beam Width

[0045] A dish antenna of two-foot diameter projects a half-power beamwidth of about 0.36 degrees at 94 GHz. The full-power beamwidth (tofirst nulls in antenna pattern) is narrower than 0.9 degrees. Thissuggests that up to 400 independent beams could be projected azimuthallyaround an equator from a single transmitter location, without mutualinterference, from an array of 2-foot dishes. At a distance of fivemiles, two receivers placed 400 feet apart can receive independent datachannels from the same transmitter location. Conversely, two receiversin a single location can discriminate independent data channels from twotransmitters ten miles away, even when the transmitters are as close as400 feet apart. Larger dishes can be used for even more directivity.

Backup Microwave Transceiver Pair

[0046] During severe weather conditions data transmission quality willdeteriorate at millimeter wave frequencies. Therefore, in preferredembodiments of the present invention a backup communication link isprovided which automatically goes into action whenever a predetermineddrop-off in quality transmission is detected. A preferred backup systemis a microwave transceiver pair operating in the 10.7-11.7 GHz band.This frequency band is already allocated by the FCC for fixedpoint-to-point operation. FCC service rules parcel the band intochannels of 40-MHz maximum bandwidth, limiting the maximum data rate fordigital transmissions to 45 Mbps full duplex. Transceivers offering thisdata rate within this band are available off-the-shelf from vendors suchas Western Multiplex Corporation (Models Lynx DS-3, Tsunami 100BaseT),and DMC Stratex Networks (Model DXR700 and Altium 155). The digitalradios are licensed under FCC Part 101 regulations. The microwaveantennas are Cassegrain dish antennas of 24-inch diameter. At thisdiameter, the half-power beamwidth of the dish antenna is 3.0 degrees,and the full-power beamwidth is 7.4 degrees, so the risk of interferenceis higher than for MMW antennas. To compensate this, the FCC allocatestwelve separate transmit and twelve separate receive channels forspectrum coordination within the 10.7-11.7 GHz band. Sensing of amillimeter wave link failure and switching to redundant microwavechannel is an existing automated feature of the network routingswitching hardware available off-the-shelf from vendors such as Cisco,Foundry Networks and Juniper Networks. The reader should understand thatin many installations the provision of a backup system will not bejustified from a cost-benefit analysis depending on factors such ascosts, distance between transmitters, quality of service expected andthe willingness of customers to pay for continuing service in the worseweather conditions.

Narrow Beam Width Antennas

[0047] The narrow antenna beam widths afforded at millimeter-wavefrequencies allow for geographical portioning of the airwaves, which isimpossible at lower frequencies. This fact eliminates the need for bandparceling (frequency sharing), and so enables wireless communicationsover a much larger total bandwidth, and thus at much higher data rates,than were ever previously possible at lower RF frequencies.

[0048] The ability to manufacture and deploy antennas with beam widthsnarrow enough to ensure non-interference, requires mechanicaltolerances, pointing accuracies, and electronic beam steering/trackingcapabilities, which exceed the capabilities of the prior art incommunications antennas. A preferred antenna for long-rangecommunication at frequencies above 70 GHz has gain in excess of 50 dB,100 times higher than direct-broadcast satellite dishes for the home,and 30 times higher than high-resolution weather radar antennas onaircraft. However, where interference is not a potential problem,antennas with dB gains of 40 to 45 may be preferred.

[0049] Most antennas used for high-gain applications utilize a largeparabolic primary collector in one of a variety of geometries. Theprime-focus antenna places the receiver directly at the focus of theparabola. The Cassegrain antenna places a convex hyperboloidal secondaryreflector in front of the focus to reflect the focus back through anaperture in the primary to allow mounting the receiver behind the dish.(This is convenient since the dish is typically supported from behind aswell.) The Gregorian antenna is similar to the Cassegrain antenna,except that the secondary mirror is a concave ellipsoid placed in backof the parabola's focus. An offset parabola rotates the focus away fromthe center of the dish for less aperture blockage and improved mountinggeometry. Cassegrain, prime focus, and offset parabolic antennas are thepreferred dish geometries for the MMW communication system.

[0050] A preferred primary dish reflector is a conductive parabola. Thepreferred surface tolerance on the dish is about 15 thousandths of aninch (15 mils) for applications below 40 GHz, but closer to 5 mils foruse at 94 GHz. Typical hydroformed aluminum dishes give 15-mil surfacetolerances, although double-skinned laminates (using two aluminum layerssurrounding a spacer layer) could improve this to 5 mils. The secondaryreflector in the Cassegrainian geometry is a small, machined aluminum“lollipop” which can be made to 1-mil tolerance without difficulty.Mounts for secondary reflectors and receiver waveguide horns preferablycomprise mechanical fine-tuning adjustment for in-situ alignment on anantenna test range.

Flat Panel Antenna

[0051] Another preferred antenna for long-range MMW communication is aflat-panel slot array antenna such as that described by one of thepresent inventors and others in U.S. Pat. No. 6,037,908, issued Mar. 14,2000, which is hereby incorporated herein by reference. That antenna isa planar phased array antenna propagating a traveling wave through theradiating aperture in a transverse electromagnetic (TEM) mode. Acommunications antenna would comprise a variant of that antennaincorporating the planar phased array, but eliminating thefrequency-scanning characteristics of the antenna in the prior art byadding a hybrid traveling-wave/corporate feed. Flat plates holding a5-mil surface tolerance are substantially cheaper and easier tofabricate than parabolic surfaces. Planar slot arrays utilizecircuit-board processing techniques (e.g. photolithography), which areinherently very precise, rather than expensive high-precision machining.

Coarse and Fine Pointing

[0052] Pointing a high-gain antenna requires coarse and finepositioning. Coarse positioning can be accomplished initially using avisual sight such as a bore-sighted rifle scope or laser pointer. Theantenna is locked in its final coarse position prior to fine-tuning. Thefine adjustment is performed with the remote transmitter turned on. Apower meter connected to the receiver is monitored for maximum power asthe fine positioner is adjusted and locked down.

[0053] At gain levels above 50 dB, wind loading and tower or buildingflexure can cause an unacceptable level of beam wander. A flimsy antennamount could not only result in loss of service to a wireless customer;it could inadvertently cause interference with other licensed beampaths. In order to maintain transmission only within a specific “pipe,”some method for electronic beam steering may be required.

Beam Steering

[0054] Phased-array beam combining from several ports in the flat-panelphased array could steer the beam over many antenna beam widths withoutmechanically rotating the antenna itself. Sum-and-difference phasecombining in a mono-pulse receiver configuration locates and locks onthe proper “pipe.” In a Cassegrain antenna, a rotating, slightlyunbalanced secondary (“conical scan”) could mechanically steer the beamwithout moving the large primary dish. For prime focus and offsetparabolas, a multi-aperture (e.g. quad-cell) floating focus could beused with a selectable switching array. In these dish architectures,beam tracking is based upon maximizing signal power into the receiver.In all cases, the common aperture for the receiver and transmitterensures that the transmitter, as well as the receiver, is correctlypointed.

[0055] The microwave backup links operate at approximately eight timeslower frequency (8 times longer wavelength) than the millimeter wavelink. Thus, at a given size, the microwave antennas have broader beamwidths than the millimeter-wave antennas, again wider by about 8 times.A typical beam width from a 2-foot antenna is about 7.5 degrees. Thisangle is wider than the angular separation of four service customersfrom the relay tower and it is wider than the angular separation of thebeam between the relay station and the radio antenna. Specifically, theminimum angular separation between sites serviced from the relay stationis 1.9 degrees. The angular separation between receivers at radioantenna tower 79 and relay station 76 is 4.7 degrees as seen from atransmitter at facility 70. Thus, these microwave beams cannot beseparated spatially; however, the FCC Part 101 licensing rules mandatethe use of twelve separate transmit and twelve separate receive channelswithin the microwave 10.7 to 11.7 GHz band, so these microwave beams canbe separated spectrally. Thus, the FCC sponsored frequency coordinationbetween the links to individual sites and between the links to the relaystation and the radio antenna will guarantee non-interference, but at amuch reduced data rate. The FCC has appointed a Band Manager, whooversees the combined spatial and frequency coordination during thelicensing process.

Other Wireless Techniques

[0056] Any millimeter-wave carrier frequency consistent with U.S.Federal Communications Commission spectrum allocations and servicerules, including MMW bands currently allocated for fixed point-to-pointservices at 57-64 GHz, 71-76 GHz, 81-86 GHz, and 92-100 GHz, can beutilized in the practice of this invention. Likewise any of the severalcurrently-allocated microwave bands, including 5.2-5.9 GHz, 5.9-6.9 GHz,10.7-11.7 GHz, 17.7-19.7 GHz, and 21.2-23.6 GHz can be utilized for thebackup link. The modulation bandwidth and modulation technique of boththe MMW and microwave channels can be increased, limited again only byFCC spectrum allocations. Also, any flat, conformal, or shaped antennacapable of transmitting the modulated carrier over the link distance ina means consistent with FCC emissions regulations can be used. Horns,prime focus and offset parabolic dishes, and planar slot arrays are allincluded.

[0057] Transmit power may be generated with a Gunn diode source, aninjection-locked amplifier or a MMW tube source resonating at the chosencarrier frequency or at any sub-harmonic of that frequency. Source powercan be amplitude, frequency or phase modulated using a PIN switch, amixer or a bi-phase or continuous phase modulator. Modulation can takethe form of simple bi-state AM modulation, or can involve more than twosymbol states; e.g. using quantized amplitude modulation (QAM).Double-sideband (DSB), single-sideband (SSB) or vestigial sideband (VSB)techniques can be used to pass, suppress or reduce one AM sideband andthereby affect bandwidth efficiency. Phase or frequency modulationschemes can also be used, including simple FM, bi-phase, or quadraturephase-shift keying (QPSK). Transmission with a full or suppressedcarrier can be used. Digital source modulation can be performed at anydate rate in bits per second up to eight times the modulation bandwidthin Hertz, using suitable symbol transmission schemes. Analog modulationcan also be performed. A monolithic or discrete-component poweramplifier can be incorporated after the modulator to boost the outputpower. Linear or circular polarization can be used in any combinationwith carrier frequencies to provide polarization and frequency diversitybetween transmitter and receiver channels. A pair of dishes can be usedinstead of a single dish to provide spatial diversity in a singletransceiver as well.

[0058] The MMW Gunn diode and MMW amplifier can be made on indiumphosphide, gallium arsenide, or metamorphic InP-on-GaAs. The MMWamplifier can be eliminated completely for short-range links. Themixer/downconverter can be made on a monolithic integrated circuit orfabricated from discrete mixer diodes on doped silicon, galliumarsenide, or indium phosphide. The phase lock loop can use amicroprocessor-controlled quadrature (I/Q) comparator or a scanningfilter. The detector can be fabricated on silicon or gallium arsenide,or can comprise a heterostructure diode using indium antimonide.

[0059] The backup transceivers can use alternative bands 5.9-6.9 GHz,17.7-19.7 GHz, or 21.2-23.6 GHz; all of which are covered under FCC Part101 licensing regulations. The antennas can be Cassegrainian, offset orprime focus dishes, or flat panel slot array antennas, of any sizeappropriate to achieve suitable gain.

Prefabricated Cellular Base Station

[0060] In a preferred embodiment a prefabricated base station isprovided for quick and easy installation on commercial buildingroof-tops. All of the components of the base station as described aboveare pre-assembled in the prefabricated station. These components includethe cellular transceiver for communication with users and the millimeterwave transceiver for operation as a part of the trunk line as describedabove.

[0061] While the above description contains many specifications, thereader should not construe these as a limitation on the scope of theinvention, but merely as exemplifications of preferred embodimentsthereof. For example, the 71.0-76 GHz and 81.0 to 86 GHz bands utilizedfor point to point trunk lines would work very well in the aboveapplications. The present invention is especially useful in thoselocations where fiber optics communication is not available and thedistances between communications sites are less than about 15 miles butlonger than the distances that could be reasonably served with freespace laser communication devices. Ranges of about 1 mile to about 10miles are ideal for the application of the present invention. However,in regions with mostly clear weather the system could provide goodservice to distances of 20 miles or more. Accordingly the reader isrequested to determine the scope of the invention by the appended claimsand their legal equivalents, and not by the examples given above.

What is claimed is:
 1. A cellular communications system providingwireless communication with system users and having a wirelessmillimeter wave trunk line for communicating with a telephonecommunication office, said system comprising: A) a plurality of cellularbase stations each of said base stations serving a communication cell,each of said base stations comprising: 1) a low frequency transceiverfor communicating with users within said cell at a cell phone radiofrequency lower than 3 GHz, 2) a high frequency transceiver forcommunicating with other base stations and the communications office asa part of said trunk line at a trunk line frequency higher than 60 GHz,said high frequency transceiver having up-converting equipment forconverting said cell phone radio frequency to said trunk line frequencyand down-converting equipment for down converting said trunk linefrequency to said cell phone frequency. B) at least one communicationstelephone office high frequency transceiver operating as a part of saidtrunk line in communication with said plurality of high frequencytransceivers and the communications office at a frequency higher than 60GHz.
 2. A cellular communication system as in claim 1 wherein each ofsaid base station transceivers is configured to transmit to and receivefrom a second site through atmosphere digital information at rates inexcess of 1 billion bits per second during normal weather said firsttransceiver comprising an antenna producing a beam having a half-powerbeam width of about 2 degrees or less.
 3. A system as in claim 1 whereinone of said high frequency transceivers are configured to transmit atfrequencies in the range of about 92.3 to 93.2 GHz and to receiveinformation at frequencies in the range of about 94.1 to 95.0 GHz.
 4. Asystem as in claim 1 and further comprising a back-up transceiver systemoperating at a data transmittal rate of less than 155 million bits persecond configured continue transmittal of information between said firstand second sites in the event of abnormal weather conditions.
 5. Asystem as in claim 4 wherein said backup transceiver system is amicrowave system.
 6. A system as in claim 4 wherein said backuptransceiver system is configured to operate in the frequency range of10.7 to 11.7 GHz.
 7. A system as in claim 4 wherein said backuptransceiver system is configured to operate in the frequency range of5.9 to 6.9 GHz.
 8. A system as in claim 4 wherein said backuptransceiver system is configured to operate in the frequency range of 13to 23 GHz.
 9. A system as in claim 1 wherein both said high frequencytransceivers are equipped with antennas providing a gain of greater than40 dB.
 10. A system as in claim 9 wherein at least one of said antennasis a flat panel antenna.
 11. A system as in claim 9 wherein at least oneof said antennas is a Cassegrain antenna.
 12. A system as in claim 9wherein at least one of said antennas is a prime focus parabolicantenna.
 13. A system as in claim 9 wherein at least one of saidantennas is an offset parabolic antenna.
 14. A system as in claim 1wherein said high frequency transceivers are capable of transmitting andreceiving at rates in excess of 1 billion bits per second and theantennas of both systems are configured to produce beam havinghalf-power beam widths of about 0.36 degrees or less.
 15. A system as inclaim 1 wherein one of said high frequency transceivers are configuredto transmit at frequencies in the range of about 71-76 GHz.
 16. A systemas in claim 1 wherein one of said high frequency transceivers areconfigured to transmit at frequencies in the range of about 81-86 GHz.